The imaging of textiles and other materials using thermal transfer of sublimable dyes has been commercially practiced for more than 50 years. Creating the images to be transferred has been accomplished using established imaging technologies such as off-set press, silk screen, and ink jet methods, or the like. The image is usually formed on paper using inks containing sublimable dye colorants. The transfer paper decals are then brought into contact with the textile or other material to be decorated and with the application of heat, about 100.degree. to 300.degree. C., and pressure, to assure intimate contact between the donor and receptor, the dye is vaporized and transferred as a gas, imagewise, to the receptor. Thus, a permanent image is formed.
This technology is widely practiced and well understood. With the introduction of laser printers for use with personal computers in the mid 1980's, attempts were made to incorporate thermal transfer sublimable dyes into the toners used in these printers with only limited success. The printers were intended to image in only one color, particularly black. However, when a toner was properly formulated for this application and a sublimable dye was incorporated into the toner, images could be formed which could then be thermally transferred by the application of sufficient heat to vaporize the dye. By this method, a single color image could be formed. Since many of these laser printers used replaceable cartridges to carry the toner to form the image in this electrophotographic process, various of these special thermal transfer toners could be installed in several cartridges, including toners containing the process color dyes for cyan, magenta, and yellow color imaging. Using a color separation program on a personal computer connected to such a laser printer, a skilled operator could effectively create a color separation of a full color image and print each separation by installing in turn the appropriate cartridge containing the indicated color -cyan, magenta, or yellow. By this method, an image containing the appropriate cyan, yellow and magenta thermal transfer dyes can be stepwise constructed. Even a skilled operator, however, would require about 10 minutes to complete the stepwise process to produce one full color image. In addition, due to the multiple passes of a substrate needed to apply all of the colors, and other considerations, registration of the colors is often a problem.
More recently, process color laser printers and copiers have been introduced and have gained some commercial acceptance. However, because the computer technology needed to adapt process color to these printers is relatively new, the printers and copiers are still relatively expensive, thus hindering their widespread use to date.
Given a lack of full penetration into the available market of the print engines, there has not been much interest in the preparation of after market toners for these machines by independent producers. Therefore, to date there has been little research successfully completed regarding solving the technical problems associated with preparation of suitable color toners. This lack of interest and effort is due not only to the small, fragmented market, but even more so to the difficult technical challenge that must be addressed by one in this area, i.e., one must solve not only the problems of making a single color toner, but also the problem of making all four color toners which will function well individually while establishing and maintaining a proper color balance between the various color toners during use.
In short, early on there was no need to develop toners for full process color as there were no machines available which provided a means for printing images by this process. More recently, while the machines have become available, there is not a sufficiently large base of them installed to provide an economic incentive to the independent developer to become skilled in formulating toners for full color processing on these machines.
In order to formulate process color thermal transfer dye sublimation toner for use in one or more of the commercial color laser printers or copiers, one must achieve the following: first, one must master an understanding of the process, chemistry and requirements for functional toners for use in the aforementioned machines. Second, one must use this knowledge to develop a functional set of color balanced toners containing sublimable dyes. This necessarily requires an understanding and knowledge of the different chemistry involved in the use of sublimable dyes. Thirdly, the primary images formed using the above-mentioned toners must be suitable for making secondary images on a suitable receptor substrate using conventional dye sublimation thermal transfer methods, i.e., only the dye must transfer, and the toner must stay on the transfer sheet.
A thorough study of the existing color toner technology reveals that the majority of color toner systems in use today are formulated with low melt viscosity, mostly linear polyester resins. It has been found that toners formulated to meet the imaging requirements of the standard toners, as used in the popular commercially successful color laser printers and copiers, which generally employ such low viscosity, often polyester, polymers, are too tacky and sticky for use in making dye sublimation transfer sheets used at the elevated temperatures needed to cause vaporization of the dyes.
Given the foregoing, it becomes clear, as stated above, that there are three basic problems confronting the skilled artisan attempting to formulate a commercially useful set of process color thermal transfer dyes sublimation toners. To reiterate these concerns, first, one must have a knowledge of practical toner formulating, an understanding of color electrophotography and an understanding of color toner technology. Second, one must successfully incorporate thermal transfer sublimation dyes into a totally functional set of toners for use in a commercial color laser printer or copier. And third, one must formulate the toners to function as dye sublimation thermal transfer decals without mass transfer of the toner resin to the secondary substrate.
The most difficult problem is that relating to transferring only the dye to the secondary substrate. For nearly two decades toners have been formulated to retard their inherent tendency to adhere to hot surfaces. At least three approaches to solving this problem are in use in conventional toners today. As taught by U.S. Pat. No. Re31,072 to Jadwin, high molecular weight and especially cross linked polymers may be used. Another means of solving this problem involves the incorporation of internal lubricating agents, such as waxes. A third solution is the incorporation of inert, preferably organic fillers, such as metal oxides, carbonates and the like, to act as flatting agents and which retard tack in most resins.
The incorporation of two or more of these approaches is especially effective in preventing mass transfer of the toner to the receptor substrate during sublimation transfer of the dye image. The use of inert fillers is particularly well suited to monochrome sublimation toners which have been monocomponent magnetic toners. These toners, which are formulated for use in certain machines, must contain from 25% to 60% by weight magnetite or other suitable magnetic material or pigment in order to properly function in the machine. They typically also contain moderately high molecular weight or even cross linked polymers, and also from about 2% to about 6% of a wax component. While these toners may inadvertently solve the mass transfer problem mentioned above, they do not lend themselves to use in process color printing because of their inherent dark color, which results from the necessary inclusion of magnetic pigments, which are dark colored materials. This coloring affect of the magnetic pigments also detracts from the high degree of transparency which is desirable for a proper blending of the primary colors to produce the various secondary colors. Thus, the incorporation of inert filler materials, most of which are dark colored or opaque, is not suited to full color process imaging.
Attempts at the inclusion of sublimable dyes into toners are seen for example, in U.S. Pat. Nos. 5,555,813 and 4,536,462. U.S. Pat. No. 5,555,813 describes a toner containing a sublimable dye intended for use in the preparation of images to be transferred to a secondary substrate. This patent teaches, however, that in order to transfer the sublimable dye component a molecular sieve, preferably a zeolite, must be included in the toner composition to assist in dye transfer. The molecular sieve retains the dye in its voids and then transfers the dye upon heating at elevated temperatures. U.S. Pat. No. 4,536,462 also discusses the use of sublimation dyes to prepare toner compositions. The toner is a monochrome, magnetic toner product. This teaching requires the inclusion of a surfactant in the composition in order to achieve good image development. As these patents demonstrate, the inclusion of sublimation dyes into toners for color processing requires special considerations.
Transfer sheet printing may be enhanced by the use of sublimation dye colorants. The resins historically used in the process printing and copying industry, however, are not suitable for use when the dye component to be transferred by the process is a sublimation dye. These dyes require the application of high temperatures in order to sublime. The linear polymer resins normally included in toner products, to assure proper colorant dispersion and image quality, and which are well suited for today's most popular printers and copiers, become very tacky and sticky at the elevated temperatures required to sublime the disperse dyes, making clean transfer of the dye alone impossible.
It has remained for this invention to provide toners which meet the above mentioned requirements of excellent functionality as electroscopic toners in various commercial color laser printers and copiers, which contain a balanced set of sublimable dyes, and which resist mass transfer of the toner resin system to the receptor substrate during dye sublimation transfer.